●Special Features : Water Management

Management of Dam Reservoirs withConsideration for Downstream Environments
FUJITA Koh-ichi (Head), ONUMA Katsuhiro (Senior Researcher), SUZUKI Hiroyuki (Researcher)River Environment Division, Environment Department

1. Introduction
The dam reservoirs of Japan play indispensable roles inour everyday lives by providing for both stability of watersupplies and for ﬂood control. At the reservoirs themselves,
we are working to handle several challenges, including thedecline of water quality as a result of eutrophication, cold
efﬂuent, prolonged periods of turbidity, etc, using methodsof aeration, deep aeration facilities, selectable intake,selective intake works and other techniques. The use ofsuch facilities to improve water quality has demonstrated
clear beneﬁ ts at many dam reservoirs, for example thesuppression of plankton growth et al.
However, the conventional operation of such facilities
has less focused on what should be preserved in thedownstream environment or on what environmental
purposes there might be, than on problems and issues
raised by the managers of rivers and dams and by thepeople living in the basins. In these areas, we can expect tobetter and more effectively utilize our facilities. This study
commenced in FY 2004 to set objectives for reservoirs anddownstream environments, effectively manage reservoirs toreach those goals, and prepare proposals for the operation offacilities to improve water quality and to restore downstream
environments, in what is essentially, a "Guide (to Paths toEnvironmental Improvement)."

with river water inﬂows. With these achievements, clearlydeﬁned purposes are integral to making these measuresmore effective. Therefore, based on the results of (1) above,
we must give consideration to the necessary river water
volumes (including ﬂ uctuations) and the water quality
desired to preserve river ecology, as well as environmental
goal setting technology required for reservoirs followed bydownstream rivers.
⑶ Proposed Operations Systems for Reservoirs,
River Flows, Downstream Environments① Studies of Water Quality Improvement Facilities forReservoirs
We must consider how to best operate the water quality
improvement facilities that have already been installed. For
example, we should put forward speciﬁc improvements inoperating the water quality improvement facilities installed
at reservoirs, and consider new operating policies where theimplementation of water quality measures for river ﬂows isfeasible.
Using the results from such applications and with abetter understanding of what improvements can be obtainedwith the facilities now installed at our reservoirs, we canclearly deﬁne what improvements can be expected in ourcurrent facilities if we install new facilities. With theseresults, we can constantly monitor conditions to determine if
utilizing the current facilities is sufﬁcient or not, or whether
there is a need to install new facilities to achieve the set
purposes or not.
② Developing Reservoir Water Management Systems
Using Rainfall Prediction
Current reservoir management systems use real time
calculations. By including the use of forecasts for levelsof precipitation, temperature, and sunlight hours to theseﬁ gures, we can better estimate the effects of changes
in water volume and water quality on reservoirs anddownstream river ﬂows. This would enable us to prepare
countermeasures in advance and operate reservoirs more inaccord with the changes. We are now studying the feasibilityof "enhancing management sophistication" in this manner.
③ Studies Reviewing Monitoring and Inspection Items
Based on the results, we can then consider what aspects
should be monitored in detail, what items or factors should
be added to our inspections, and what cases call for further
study (e.g. further research may be required where there
are signiﬁcant ﬂuctuations in reservoir water level or during
the algae bloom season).

2. Content and Framework of Study⑴ Current Status of Reservoirs and Downstream AreasFactors core to implement the study include anunderstanding of past environmental conditions of the area
and of current conditions of the reservoirs and downstream
areas (to include downstream ecology, the course of theriver, and physical environment factors such as riverbed
composition). While each reservoir is routinely inspected
at this time, more detailed studies for better understandingof the water movement (ﬂow) and changes in water quality
is required in some cases. For example, observation points
should be positioned, and the range of vertical monitoringthat is undertaken should be expanded. Also, monitoring andother activities are not systemically conducted for sectionsof the downstream areas, which is integral to deﬁning theexisting issues through research and observation.
⑵ Reviews of Goal Setting Methods for Reservoir andDownstream River Environment
Consideration for downstream river environments
is apparent in some of the measures already in place atseveral reservoirs, such as ﬂushing discharge and draining
water from thermal stratiﬁcation layers, much as is done

3. Technical Issues in Implementing these Studies
In terms of speciﬁ c methods to handle the issues,
32Analysis of present conditions

Application of existing result Article 3

Article 2-(3)Article 2-(1)Application of simulation model for reservoir flow
Simulation with consideration of watershed
Understanding of phytoplankton behavior in reservoir

Proposal and implementation of technology

Field survey of reservoir
and downstream river
Outlining the specificissuesClear goals Article 2-(2)Consideration for water quality improvement

Collecting existing
study results

Water circulation of reservoir
Downstream river environment

Water quality measures for watersheds andinflow river/Utilization of weather forecastingMore effective Flushing Discharge

Request from local
residents
Comparison between
the past and the present

Feedback

Outlining improvement of existing facilities
for water quality improvement

Implementation of measures for watershed and inflow river
Operation technology introducing rainfall forecasting
Implementation of Flushing Flow
Operation of more effective facilities for water quality
improvement

Monitoring
Improvement of existing monitoring
technology

Formulationof guidance●Special Features : Water Management

Article 4

Goal Amendment and updates
Evaluation of implemented technology

Figure 1 The Overall Research Flow (Numbers in table correspond to report section numbers)

we have developed simulation models accompanied byapplicability evaluation techniques. These are focused onachieving environmental purposes and lead to more effective
measures by allowing the integration of already known data
into the overall study. Some primary issues in obtaining theinformation necessary from the study are outlined below.
⑴ Point 1 - Reservoir Flows
This study includes plans to use simulation models
to outline changes in water quality, ﬂ ow behaviors andpatterns, and turbidity. We are currently working on thedevelopment of simulations of reservoir water ﬂows thatallow expression of the effects provided by aeration facilities.
By developing the simulation in accord with different uses,
we expect to be able to forecast and quantitatively express
the resultant effects (e.g., operation technology for new
aeration facility).
⑵ Point 2 - Relationships of Reservoirs and theirSurroundings
In addition to water quality improvement measures forreservoirs, the study also focuses on the watershed and willconsider in the basin water quality improvement measures.
We are now working creating models to allow simulationof different qualities of inﬂuent water, and the impact onreservoir and downstream river environments based on
the land use data from a GIS database on the upstream anddownstream watersheds.
⑶ Point 3 - Plankton Movements in Reservoirs
The effort to inhibit massive phytoplankton bloom isconcentrated on experiments using aeration facilities inreservoirs. Current facility operations focus on the increaseor decline of algae that appear with changes in water quality,
primarily in the reservoir area. We know, however, from past
study ﬁndings that zooplankton appears to have a signiﬁcant
inﬂuence on the increase and decrease of phytoplankton.
Therefore, it may be possible to provide more effective
facility operations by monitoring phytoplankton andzooplankton separately and better understanding theirmechanisms of increase and decline.

will continue by concentrating on on-site applications, andby monitoring the results, we will be able to constantly ﬁndpoints for improvement. We consider the results obtained
through these processes to provide indicators valuable forthe environmental management of the overall river systemincluding basins.

4. The Implementation and Use of Study Findings
The study simulations will permit us to determine which
operating technologies allow the most effective use offacilities, and to apply these results to reservoirs. The study*1: Effects of Flushing Discharge
Using water fl ow speed controls to discharge larger volumesof water than usual over abbreviated time spans, is expected
to refresh the river and improve the dam downstream riverenvironment.

5. Conclusion
Dams have made great contributions to ﬂood mitigationand to secure water resources. However, at the sametime, they have also fallen under critical scrutiny for their
negative impacts on river environments. In future, wemust not only ﬁ nd ways to alleviate the impact of dams
on river environments but also ascertain what is the bestusage of dams to establish desired river environments. The
study, with its actual onsite results and achievements, willcontribute toward better harmony between dams and theirenvironments.

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